4.Energetics Flashcards
Define enthalpy change
The heat energy change measured under conditions of constant pressure.
Define standard enthalpy change
The heat energy change measured under standard conditions (100kPa and 298 K)
Define standard molar enthalpy of formation
The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions (100kPa and 298 K), all reactants and products being in their standard states
Define standard molar enthalpy of combustion
The enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions (100kPa and 298 K), all reactants and products being in their standard states
What is the equation for enthalpy change
q=mcAt
enthalpy change = mass of substance (g) x SHC (Jg-1K-1) x temperature change (K)
State Hess’s Law
The enthalpy change for a reaction is the same whatever route is taken from the reactants to products
What is the enthalpies of elements
The enthalpies of all elements in their standard states (state which they exist at 298K and 100kPa) are taken as zero
What is bond dissociation enthalpy
The enthalpy change requires to break a covalent bond with all species in the gaseous state
What is mean bond enthalpy
The average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds
Why do bond dissociation enthalpies range between molecules
Bonds differ slightly in different molecules and therefore have different bond dissociation enthalpies
